Process and apparatus for dewatering controlled by monitoring light scattered by supernatant

ABSTRACT

A sedimentation or other dewatering process conducted on a suspension is controlled by filling a body (1) that is open at its top end (2) with the suspension, establishing quiescent conditions and allowing the suspension to settle to form a supernatant layer at the top end (2), generating light from an assembly (14) and measuring the amount of scattered light (19) from the supernatant layer by a collector (21) and utilizing the measured amount of scattered light to control the dewatering.

The present invention relates to the control of processes for dewateringsuspensions, especially coagulation and/or flocculation followed bysedimentation, in response to measurement of turbidity.

Various systems are known for determining the turbidity of a liquid.Many of the systems involve filling a cell of glass or transparentplastic and determining the optical properties of the cell filled withthe liquid. Such systems in which optical properties are determinedusing an arrangement where a transparent material is in contact with theliquid all suffer from the disadvantage that contamination of thetransparent material will influence the results.

A system that avoids this disadvantage is the Surface ScatterTurbidimeter SS6 manufactured by the Hach Company (see U.S. Pat. No.3,309,956). In this system there is a upwardly extending elongate bodydefining a longitudinal central bore and having an open upper end, afeed positioned to feed suspension continuously into the body below theupper end, a collector positioned around the upper end to collectsuspension that overflows from the upper end while defining a liquidsurface at the open upper end, a light source positioned to direct a rayof light to strike this liquid surface at an angle such that light isscattered from it, and a collector positioned to receive the scatteredlight. In use, the suspension under test is forced to flow continuouslyupwardly through the body so as to provide a continuously replenishedfresh layer of suspension at the upper surface which is typical of thewhole body of the suspension, and the scattering of light from thistypical upper surface is a measure of the turbidity of the suspensionthat is being tested. In order that measurable results can be achievedit is, of course, necessary that the suspension should be dilute andhave adequate light scattering properties. This system would beinoperable if used with a suspension that did not have adequate lightscattering properties.

There are many industrial processes where a suspension is dewatered by,for instance, sedimentation, filtration or centrifugation. Examplesinclude sewage sludge, papermaking thinstocks and thickstocks, andinorganic suspensions. It is standard practice to flow such a suspensionthough a service line towards a dewatering plant where the suspension isdewatered, and to flocculate or otherwise chemically modify thesuspension prior to dewatering by adding polymeric flocculant or otherchemical dewatering modifier by appropriate chemical dosing equipment.Optimum dewatering depends on the nature of the suspension, but thistends to be variable. Accordingly various techniques have been developedfor controlling the dewatering, either manually or automatically, inresponse to measurement of some physical property of the suspension.

In particular, it is known to use supernatant or filtrate turbiditymeasurements for monitoring and controlling sedimentation or otherdewatering processes, especially when the clarity of the supernatant orfiltrate is important. Usually an optimum turbidity is determined for aparticular process by running plant trials of the process underdifferent conditions, typically with different amounts or types ofdewatering treatment chemical. Subsequently, when the process is runcommercially, the process conditions are adjusted to maintain theturbidity at the optimum predetermined level.

In a typical process, the suspension to be dewatered flows along aconventional service flow pipeline, past a dosing point at which adewatering treatment chemical is added to the suspension to a dewateringplant at which it is allowed to settle. Settling of the suspensionusually takes place in a large sedimentation tank and may take two hoursor more. After settling, turbidity measurements are taken on thesupernatant. If this is done using a turbidimeter submerged in the tankor after the tank, it is necessary to wait for settlement before usefulturbidity measurements can be taken. This means that the overalltreatment process will have advanced considerably before any change indosing of treatment chemical can be effected in response to the measuredturbidity. Therefore the treatment process may run inefficiently forsome periods of time and may never reach full efficiency.

Measurement may alternatively be by removing a sample of the suspensionfrom the pipeline and placing it in a stand-alone turbidimeter which isdesigned to generate in a shorter period a sufficient volume ofsupernatant on which turbidity measurements can be conducted. Althoughthe necessary settlement period can be less than the two or more hoursgenerally required in a settlement tank, in practice it is generallystill necessary to leave the suspension for quite a long time (e.g. halfan hour or more) to allow sufficient settlement to permit meaningfulturbidity measurements to be conducted by available apparatus.Accordingly the overall treatment process will still have advancedbefore the need to modify the dewatering conditions has become apparentand so the process is rather inefficient. Also, sampling from theservice line to the laboratory analytical apparatus is inconvenient andmakes automatic control of the process difficult to achieve.

Many of the suspensions that come into question have a tendency to foulapparatus that they contact. Accordingly a problem when the turbidimeteris submerged in the suspension, (for instance in or after thesedimentation tank) is that it is likely to suffer fouling with theresult that it has to be cleaned frequently and normally will have to beremoved from the process to permit adequate cleaning. Stand-aloneturbidity meters can be cleaned in position, but the careful cleaningneeded to obtain reliable results is inconvenient.

It would be desirable to provide a way of monitoring, and controlling,sedimentation or other dewatering processes using turbidity measurementsand which has a quicker response time than prior art processes, andwhich therefore allows substantially immediate alteration of the processin accordance with the current properties of the suspension. It wouldalso be desirable to provide a process in which there is less risk ofthe turbidimeter measurements being rendered unreliable by fouling. Itwould also be desirable to provide such a process which could beconducted substantially on-line with the service flow of suspension tobe sedimented.

According to the present invention a process for monitoring dewateringof a suspension comprises the steps of

providing an upwardly extending elongate body defining a longitudinalcentral bore and having an open upper end,

filling the body with suspension until a surface of suspension is formedacross the open end,

establishing quiescent conditions and allowing the suspension to settleto form a supernatant layer,

directing a ray of light to strike the surface at an angle such thatlight is scattered by the supernatant layer,

positioning a collector to receive light scattered by the supernatantlayer,

measuring by the collector the amount of scattered light it receivesfrom the supernatant layer, and

utilising the amount of scattered light measured by the collector tocontrol dewatering of the suspension.

The process is extremely rapid to operate since meaningful measurementscan be obtained as soon as a thin supernatant layer has formed bysettlement. Since the optical measurements are conducted on the liquidsurface, rather than through a glass cell, inaccuracies due to foulingof a cell containing the liquid are avoided.

When the upper liquid surface is first formed across the open end, thetop part of the liquid will have the same composition as the remainderof the liquid in the body. Usually the optical density of the suspensionis too high to give meaningful turbidity values at this stage fromwithin the suspension, and even if a meaningful turbidity value isobtained on the freshly formed surface of the suspension, the value willbe of the suspension itself. This is of little or no assistance in theinvention, where the need is to determine the turbidity value of thesupernatant obtained upon settling the suspension. Depending upon thenature of the suspension, settlement may either lead to the formation ofa relatively clear mud line between supernatant of substantially uniformsolids content above the line and sediment below the line, or it maylead to a supernatant through which solid content increases graduallydownwards. Either way, after allowing settling to occur the solidscontent at the top of the column of liquid will be less than at lowerpositions.

The dewatering process in the invention can be controlled either inresponse to a constant turbidity value that is recorded or in responseto the relationship between time and the change in turbidity.

Light that strikes the top of the liquid, generally near the centre ofthe open liquid surface, will be scattered by the particles itencounters. A collector is positioned above the settled layer to collectlight that is scattered generally at approximately right angles to thecentre of the surface. Before settling starts, all the scattering oflight will be from the unsettled suspension. Once settling has started,the scattering of light will be due to particles in the supernatant and,if the supernatant layer is thin, to scattering in the unsettledsuspension (below the mud line if there is one). As the mud line sinks,the location of the scattered light that is scattered from below the mudline will move with respect to the collector. The collector can be sopositioned that when the supernatant is deep enough none of the lightscattered from below the mud line will reach the collector. Accordingly,the amount of scattered light that reaches the collector (and thereforethe recorded or apparent turbidity) may start high, due to a substantialamount of scatter from below the mud line, but will drop to asubstantially constant value that is due solely to scatter in thesupernatant, settled, layer. Typically, the constant value is obtainedwhile the layer is still shallow, e.g. up to 10 mm or 20 mm thick.Useful measurements (e.g. of the rate of change of collected scatteredlight) can be obtained with even thinner layers.

It will thus be apparent that in some processes of the inventionsettling is allowed to occur until the amount of scattered lightrecorded by the collector is substantially constant, and this constantvalue is utilised to control the dewatering of the suspension. Thisconstant value is a function of the suspended matter in the supernatantlayer only since in such processes the collector receives substantiallyno light from the supernatant layer that was scattered from below themud line, when the value is constant. If it is desired only to rely uponthis constant value, then the collector can be at a position to receiveand measure only scattered light from within the supernatant at a chosentime after establishing quiescent conditions, this position being suchthat a substantially constant amount of scattered light is received bythe collector.

Instead of or in addition to relying upon this constant value forcontrolling the dewatering, reliance is often placed on the relationshipbetween the amount of scattered light measured by the collector and thetime after establishing quiescent conditions. For this purpose, thecollector is positioned to receive and measure light scattered fromwithin and beneath the supernatant layer at one time after establishingquiescent conditions and to receive and measure light scatted wholly ormainly from within the supernatant layer at a later time. Often, theprocess is conducted by recording initially the relationship betweentime and scatter and, subsequently by recording the substantiallyconstant value.

Since the constant value can be determined on a layer which is quitethin (e.g. under 10 mm or 20 mm thick), determination of this constantvalue can be achieved quickly after establishing quiescent conditions.Since meaningful measurements (and in particular the rate of change ofscatter) can be determined on even thinner layers, meaningful resultscan be achieved even before then. Thus, by the invention, it ispossible, after a settling period of only a few seconds or minutes, toobtain results by which the dewatering process can be controlled.

The process can be conducted as a manual, stand-alone process, with thesuspension being poured into the body and the timing and theobservations being conducted manually. Preferably, however, there issome degree of automation. Suitable novel apparatus for performing theinvention comprises

an upwardly extending elongate body defining a longitudinal central boreand having an open upper end,

a valved feed position to feed suspension into the body and means forclosing this feed,

a light source positioned to direct a ray of light to strike the surfaceat an angle such that light is scattered from the liquid at the openupper end,

a light collector positioned to receive light scattered from the liquidat the open upper end,

timing means by which the time between closing the valved feed andrecording one or more measurements of scattered light is automaticallycontrolled or recorded, and

signal means for generating a signal in response to the measurement ofscattered light.

The method can be conducted remote from a service line though which thesuspension is flowing, to generate a signal that can be transmitted to aremote dewatering process. For instance the apparatus may be used at alaboratory to which a batch of the suspension is taken and can be usedto generate a numerical or other display in print-out or other signalform that can be used manually to control the sedimentation or otherdewatering of the bulk suspension from which the sample was taken.Similarly, when the method is being conducted close to the service line,with the suspension being fed through a valved feed from the serviceline to the body, the apparatus can provide a display to assist manualcontrol of the dewatering process. Preferably, however, the apparatusincludes control means for controlling the dewatering process and thesecontrol means are constructed to be controlled automatically in responseto the amount of scattered light measured by the collector. Thus thereshould be appropriate signal means to generate a signal which controlsthe dewatering.

Suspension can be run off from the service flow either direct into theopen top of the body or into a vessel by which the suspension can betransported to the apparatus or, preferably, the body is in fluidcommunication with the service flow (e.g. a carrier duct or pipeline)through a screened valved inlet suitable for feeding the suspension fromthe pipeline into the body. Accordingly the invention can provide anon-line method of controlling the dewatering process.

Generally the suspension is flowing from a dosing pump or other stage atwhich was added a chemical dewatering modifier.

Preferably, the body is filled through a valved feed and convenientlythis valved feed leads from a service line by which suspension isflowing to a dewatering apparatus. The valved feed can discharge intothe top of the body, but preferably is a valved inlet that leads intothe body at a position below the open upper end. When a timer isincorporated with such an apparatus, the feeding of the suspension intothe body, the establishment of quiescent conditions and the measurementof scattered light are preferably all controlled automatically by thetimer.

Thus a typical sequence will involve filling the body with thesuspension, stopping the filling and thereby establishing quiescentconditions and determining the amount of scattering at a predeterminedor measured time after stopping the filling.

It is often preferred for the suspension to settle sufficient to give asupernatant layer having sufficient depth to give meaningful turbiditymeasurement of the supernatant. Accordingly the timer may be such thatthe suspension is settled for a predetermined period before somemeasurements are taken. This period may be a relatively arbitrary periodthat is known to be sufficient to allow a sufficient supernatant layerto form or it can be a period that is selected reasonably accurately,and as short as possible, having regard to the anticipated settlingproperties of the suspension.

The amount of light scattered off the surface of the suspension may bemeasured through the predetermined period (either continuously throughit or, for instance., through the later stages of it) or after thepredetermined period (for instance either a substantially instantaneousmeasurement at the end of the period or several measurements after thepredetermined period).

Instead of controlling the measurement of scatter after a predeterminedtime, the timer may be constructed to determine the time at which apredetermined amount of scattered light is measured. For instance theresult of the process might be a time value that is required to achievea particular level of turbidity, rather than a turbidity value achievedafter a particular time.

In one typical process, the suspension is settled for a predeterminedtime and the amount of light scattered off the surface of the liquidonto the collector is measured up to the predetermined time or at orafter that time and the measured amount of light is utilised to generatethe signal that is used to control the dewatering process.

In another typical process, the suspension is settled for sufficienttime to allow the rate of change of the amount of scattered light toreach a predetermined value. The amount of scattered light is measuredthroughout this period. The amount of scattered light at a predeterminedvalue for the rate of change and/or the time required to reach thischosen value is utilised to control the dewatering of the suspension.

In another typical process, the suspension is settled for sufficienttime for the amount of scattered light to reach a chosen value. Theamount of scattered light is measured through this period. The timerequired to reach this chosen value is utilised to control thedewatering of the suspension.

When utilising the time required for the amount to reach a particularvalue, it is usually preferred to utilise the time required to changefrom one predetermined value to another can be used to control thesuspension.

The upwardly extending elongate body that is to contain the testsuspension can extend in a vertical direction or in a direction thatforms an angle (for instance 20° to 60° ) to the vertical. The body istypically made of plastic. It may have dimensions of 10 to 50 cm inlength and 5 to 20 cm in width. The top of the body preferably has agenerally flat horizontal lower edge which can conveniently serve as aweir over which the suspension can uniformly flow. Accordingly, it canbe convenient for the top of the body, and often the entire body, tohave square cross section.

Generally the elongate body extends upwards from a base which acts as asupport and may close the bottom end of the body.

The body may be filled with suspension either by pumping or otherwisedirecting flow of suspension through a screened valved inlet below theupper end and generally at or near its lower end or by pouringsuspension into its upper end. Filling is continued until a surfacelayer of suspension is formed at the upper end.

It is normally preferred to conduct the filling until the suspensionoverflows the open end and generally means are provided for collectingsuspension that overflows from the open end after formation of thedesired surface of the suspension at the open end. The filling is thenstopped.

Following settlement and the measurement of the turbidity of the settledupper layer of the batch of suspension in the body, the body is emptiedand/or filled with a fresh batch of suspension for fresh measurement tobe made. The body can be emptied by draining the suspension through anoutlet in the bottom of the body. Drainage can be accelerated byconnection to a vacuum pump. If the valved inlet is in the bottom of thebody, the outlet may be the same opening as the inlet in which case aconventional two-way valve (or fill and draw valve) may be provided inthe opening. Alternatively there may be a separate drain outlet which isclosed during filling and measurement and which is opened to empty thebody. Alternatively, if the body is portable, it can simply be invertedto empty it.

After emptying, the body can be refilled or it can be emptied bysuspension in the body being flushed upwardly out of the body byup-flowing fluid introduced through an inlet at or near the base. Theup-flowing fluid can be fresh suspension or water or other wash liquor.

It is convenient for the invention to be performed utilising the novelapparatus that includes timing means that can be pre-programmed topermit a predetermined settling period after termination of the fillingof the body with the suspension and before the generation of the finalsignal that is used for controlling the dewatering process. However theinvention can also be conducted (less efficiently) using simpler formsof apparatus, for instance where the plant operator manually terminatesfilling, estimates an appropriate settling period, and then switches onthe light source and the measuring means.

The control of the dewatering process generally comprises changing theamount or type of dewatering modifier that is used to modify thedewatering properties, and which generally is added before the sample issubjected to the turbidity measurement.

The chemical dewatering modifier can be any chemical material that hasan effect on the dewatering properties, and in particular on the clarityof the filtrate or supernatant. Generally the chemical is a polymericflocculant which has a high molecular weight (e.g. intrinsic viscosityabove 4 dl/g) and is of the type that is commonly referred to as abridging flocculant, but if desired the chemical can be a low molecularweight (e.g. intrinsic viscosity below 3 dl/g), highly ionic, polymericflocculant which is more accurately generally referred to as acoagulant. Other chemicals that can be used included inorganiccoagulants such as multi-valent metal salts, natural polymers such asstarch or cationic starch, polyethylene imine, polyethylene oxide, andsome inorganic materials such as polysilisic acid and derivatives ofthis, and swelling clays, generally known as bentonites. Usually,however, a polymeric bridging or coagulant flocculant is used.

The dewatering process is generally a sedimentation stage or can be, forinstance, a filtration or centrifugation stage. When the dewateringprocess conditions are variable, e.g. the pressure of filtration or thespeed or centrifugation, such conditions of dewatering can be controlledin response to the turbidity measurement obtained in the inventioninstead of or in addition to controlling the dosing of chemicals.

The process and apparatus of the present invention are suitable for useon any suspension that is capable of settling in a useful manner. Thesolids content can be low (e.g. below 1% by weight) when the suspensionis colloidal and/or organic, but can be much higher provided thesuspended matter is such that it can settle to leave an uppersupernatant. For instance many inorganic materials will settle in thismanner when the solids content is up to 20% by weight or even more.Typical suspensions are raw sewage (e.g. 10 to 1000 ppm), secondarysewage (e.g. 500 to 5000 ppm), and pigments, non-swelling clays and coaltailings (e.g. 1 to 20%).

The invention is illustrated in the accompanying drawings in which

FIG. 1 shows a diagrammatic side view of suitable apparatus in fluidcommunication with a service flow pipeline (not to scale).

FIGS. 2 to 5 are graphs showing changes in the recorded value of theturbidity of the settled supernatant layer.

The apparatus comprises an inclined cylindrical body 1 that is open atits top end 2. An inlet line 3 including a valve 4 provides fluidcommunication between a service flow line 5 for sewage or othersuspension and the body 1. A collecting cylinder 6 surrounds the top endof the body and is positioned to collect the fluid that overflows thetop end 2 as a result of the body being filled through inlet 3. A drainline 7 leads from the base of the cylinder through a non-return valve 8back into the service line. Alternatively, it can lead to drain.

A line 9 leads from the bottom end 10 of the cylinder to a valve 11 bywhich the line can be opened or closed. If the line is to be used merelyfor drainage at the end of a process, it may be adequate merely for theline 9 and valve 11 to discharge into the service line 5 or it maydischarge to drain (not shown). Often, however, line 9 is used forbackwashing the cylindrical body 1 either with water from a supply showndiagrammatically as 12 or by pumping suspension from the line 5 by apump (not shown). Instead of or in addition to this, there may be an airline 13 by which vacuum or compressed air can be passed through the line9. If line 9 is constructed to allow suspension to be forced from line5, through line 9, into the body 1 it may be appropriate to rely on line9 for this and to omit line 3.

There is a lamp assembly 14 including a lamp 15 and a lens 16 by which aray of light 17 can be directed onto the upper surface of the liquid inthe open top end 2 of the body 1. Some of the light striking the surfaceis reflected as a reflected ray 18, some is refracted as a refracted ray20 and some is scattered by particles in the upper liquid layer asscattered light 19. A photocell 21 is positioned to receive thescattered light 19. Instead of using a photocell, other conventionallight collectors can be used such as photomultipliers and lightsensitive diodes.

Suitable apparatus can be constructed by making appropriatemodifications, for instance to the valves and to the method ofoperation, of a Surface Scatter Turbidimeter SS6 manufactured by theHach Company.

The whole apparatus conveniently is enclosed within a protective housing22.

The service line 5 typically leads from a flocculant dosing point to asedimentation tank. It may be an open duct or a closed pipe and the line3 (and/or the line 9 if it is to receive fluid from the service line 5)must naturally be connected into the line 5 in an appropriate mannersuch that fluid can pass from the line 5 into the cylindrical body 1.When the line 5 is pressurised the pressure of the fluid in the line maybe sufficient, but otherwise it may be necessary to provide appropriatepumps, not shown.

In use, suspension from the line 5 is flowed through the inlet line 3 tofill the body 1 and to overflow from the top end 2 of the body into thecylinder 6, from which it drains through the drain outlet 7 and thenon-return valve 8 back into line 5. The valve 4 is then closed and thesuspension in the cylinder 1 allowed to settle for a predetermined time.The ray of light 17 is then generated and the scattered light 19 isrecorded by the photocell either at a predetermined time or after apredetermined degree of settlement (as shown by the position of the mudline), or more usually over a predetermined time range.

There will usually be automatic timing means (not shown) for controllingthe settling time (between closing the valve 4 and making opticalmeasurement) and also for controlling and/or recording the times ofsuccessive measurements taken during settling of the same batch ofsuspension. There may also be electronic communication (not shown)between the photocell 21 and the apparatus for controlling the dosage offlocculant, and/or between the photocell and display apparatus fordisplay of the result of the measurement.

Since the sewage or other suspension does not contact the opticalapparatus, the risk of errors due to contamination of the opticalsurfaces is much less than in conventional systems.

The following is an example to further illustrate the process of thepresent invention in use as a system for monitoring (and controlling)the sedimentation of a suspension.

EXAMPLE

The suspensions under test were 1% slurries of chalk and filler clay.

300 ml of each of the slurries were treated with varying doses ofcationic polyacrylamide flocculant (available from Allied ColloidsLimited under the trade name Percol 63, Percol being a trade mark) in aliter measuring cylinder and inverted 4 times to ensure complete mixing.Each was then used to overfill the calibration cylinder in a HachSurface Scatter 6 Turbidimeter SS6. As soon as filling stopped astopwatch was started and the turbidity noted at various time intervals.

The turbidimeter was set to average the turbidity over every period of60 seconds of each test run, and so the earlier readings in the first 60seconds are not meaningful.

FIGS. 2 and 3 are graphs showing the variation of turbidity with timefor the different doses of flocculant used on the chalk and claysuspensions respectively.

FIGS. 4 and 5 are graphs showing the average turbidity for the differentdoses of flocculant used on the chalk and clay suspensions respectively.

The results show that flocculant effectiveness on chalk and clay can bepredicted by turbidity measurements using the process of the presentinvention.

I claim:
 1. A process for controlling dewatering of a suspension inresponse to determining turbidity of a supernatant obtained from thesuspension, which comprises the steps of:providing an upwardly extendingelongate body defining a longitudinal central bore and having an openupper end, filling the body with a portion of the suspension until asurface of suspension is formed across the open end, establishingquiescent conditions and allowing the suspension portion to settle toform a supernatant layer, directing a ray of light to strike the surfaceat an angle such that light is scattered by the supernatant layer,positioning a collector to receive light scattered by the supernatantlayer, measuring by the collector the amount of scattered light itreceives from the supernatant layer at different times afterestablishing quiescent conditions and thereby determining a rate ofchange of the amount of scattered light after establishing quiescentconditions, and utilizing the rate of change of the amount of scatteredlight measured by the collector to control dewatering of the suspension.2. A process according to claim 1 in which the settling is allowed tooccur until the amount of scattered light recorded by the collector issubstantially constant and this constant amount also is utilised tocontrol the dewatering of the suspension.
 3. A process according toclaim 2 in which the collector is positioned to receive and measure onlylight scattered from within the supernatant at a chosen time afterestablishing quiescent conditions, which time is such that asubstantially constant amount of scattered light is received by thecollector.
 4. A process according to claim 1 in which the suspension isa suspension that is being dewatered after addition of a chemicaldewatering modifier and the control of dewatering is by controlling theaddition of the dewatering modifier.
 5. A process according to claim 4in which the chemical dewatering modifier is a polymeric flocculentadded to the suspension before the portion is filled into the body.
 6. Aprocess according to claim 1 in which the dewatering is bysedimentation.
 7. A process according to claim 1 in which the suspensionportion that is filled into the body is flowed through a valved feed tothe body from a service line by which the suspension is flowing to bedewatered.
 8. A process according to claim 7 in which the valved feed isa valved inlet that leads into the body at a position below the openupper end.
 9. A process according to claim 7 or claim 8 in which thefeeding of the suspension portion into the body, the establishment ofquiescent conditions, and the measurement of scattered light arecontrolled automatically by timing means.
 10. A process according toclaim 1 in which the dewatering is controlled automatically in responseto the measuring of scattered light.
 11. A process according to claim 1in which the body is an inclined body and is filled with the suspensionportion until the suspension portion overflows the top end, and thefilling is then stopped thereby establishing quiescent conditions. 12.Apparatus comprisingan upwardly extending elongated body defining alongitudinal central bore having an open upper end, a valved feedpositioned to feed a portion of a suspension into the body and means forclosing the feed, a light source positioned to direct a ray of light tostrike a liquid surface at the open upper end, a light collectorpositioned to receive light scattered from the liquid at the open upperend, and means for making measurements of the amount of receivedscattered light, and in which the apparatus also comprises timing meansfor automatically controlling or recording the respective times betweenclosing the valved feed and making measurements of scattered light,means for determining the rate of change of the amount of scatteredlight over time and means for generating a signal in response to themeasurements of the amount of scattered light.
 13. Apparatus accordingto claim 12 in which the signal generating means include means forautomatically controlling the dosage of a chemical dewatering modifieradded to the suspension in response to the signal.
 14. Apparatusaccording to claim 12 including means for connecting the valved feed toa service line.
 15. Apparatus according to claim 12 including means forflushing settled suspension from the body and operable by the timingmeans after the generation of the signal.